Channel quality measurement in data transmission using hybrid arq

ABSTRACT

A hybrid ARQ technique for transmitting a data unit on a radio channel in a communication system to a receiver is provided, wherein after the encoding the data unit into a sequence of code words using an encoding parameter, a first code word is transmitted, and an ACK or NAK is received from the receiver. If a NAK is received, the next code word of the sequence is transmitted. A measurement value indicating the current channel conditions is determined by counting the NAK messages and/or evaluating the encoding parameters. The measurement values may be the overall coding rate, the average number of retransmissions per data unit, or the average number of retransmissions per code word. The transmitter may include a NAK counter. The measurement may be used for adaptation purposes.

FIELD OF THE INVENTION

The invention relates to a hybrid ARQ technique for data transmissionand in particular to a hybrid ARQ Type II/III method of operating atransmitter for transmitting a data unit on a radio channel in acommunication system to a receiver. The invention further relates to acorresponding transmitter and communication system. The invention can beapplied in mobile communication systems and is particularly applicableto cellular systems. In particular, the invention can be applied in theUniversal Mobile Telecommunication System UMTS.

BACKGROUND OF THE INVENTION

UMTS is one of the candidate technologies for next generation mobilecommunication systems, and its architecture is depicted in FIG. 1. UMTSis composed of a Core Network CN 100 which is connected with interfaceIu to the radio access network UTRAN 110 (UMTS Terrestrial Radio AccessNetwork). The UTRAN 110 is on the other side connected to the UserEquipment UE 120 with interface Uu.

As can be seen from FIG. 2, the UTRAN 110 consists of a set of RadioNetwork Subsystems RNS 200 connected to the Core Network CN 100 throughthe interface Iu. Each RNS 200 consists of a Radio Network ControllerRNC 210 which is responsible for the Handover decisions that requiresignalling to the User Equipment UE 120. Further, the Radio NetworkSubsystems RNS 200 comprise base stations (Node Bs) 220 which areconnected to the Radio Network Controller RNC 210 through an interfaceIub. Inside the UTRAN 110, the Radio Network Controllers RNC 210 of theRadio Networks Subsystems RNS 200 can be interconnected via a furtherinterface Iur. The interfaces Iu and Iur are logical interfaces. Iur canbe conveyed over a direct physical connection between the Radio NetworkControllers RNC 210 or virtual networks using any suitable transportnetwork.

The control plane signalling of Layer 3 between the User Equipment UE120 and the UTRAN 110 is handled by the Radio Resource Control RRClayer. Besides others as conveying broadcast information, establishingradio bearer, controlling radio resources, the RRC is also responsiblefor User Equipment UE measurement reporting and control of thereporting. The measurements performed by the User Equipment UE 120 arecontrolled by the RRC layer in terms of what to measure, when to measureand how to report, including both UMTS air interface and other systems.The RRC layer also performs the reporting of the measurements from theUser Equipment UE 120 to the network.

In data communications systems, error detection incorporated withAutomatic Repeat reQuest (ARQ) is widely used for error control. Themost common technique for error detection of non-real time services isbased on hybrid ARQ schemes which are a combination of ARQ and ForwardError Correction (FEC).

FEC introduces redundancy into a block of information bits of length kto form a coded block of length n, before transmission. The redundancyhelps to combat errors at the receiver.

A transmitter which performs forward error correction is depicted inmore detail in FIG. 3. The input data which are to be transmitted arefirst buffered in buffer 300. When there is data in the buffer 300 andthe transmitter is assigned a physical channel for transmission, thedata is encoded in the FEC encoder 310 thereby generating a mother code.The mother code or all the code words (code segments) of the mother codeare then forwarded to the modulator 330 and the spreader 340 (in case ofa Code Division Multiple Access CDMA system), shifted to the radiofrequency RF by the RF circuit 350 and transmitted via the antenna 360.If Type II/III ARQ (described below) is used the transmitter furthercomprises a code word buffer 320 since different code words are sent inthe retransmissions.

Turning now to the ARQ technique, the most frequently schemes used inmobile communications are the stop-and-wait (SAW) and selective-repeat(SR) continuous ARQ schemes. If an error is detected by CyclicRedundancy Check (CRC), the receiver requests the transmitter to sendadditional bits. A retransmission unit of the Radio Link Control RLClayer is referred to as protocol data unit PDU.

A transmitter arranged for being operated according to ARQ schemes isdepicted in FIG. 4. Since the transmitter has to be able to receiverequests from the receiver, the transmitter comprises a duplexer 400which allows for using one antenna 360 for transmission and reception.When the transmitter receives a signal, it shifts the signal with the RFcircuit 410 into the base band, despreads the signal in the despreader420, forwards the despread signal to the demodulator 430, and extractsan ACK/NAK signal from the demodulated data. An ACK message informs thetransmitter that the receiver was able to successfully decode thetransmitted PDU. A NAK message informs the transmitter of a decodingerror. Depending on whether the transmitter receives an ACK or a NAK,the ACK/NAK extractor 440 accesses the code word buffer 320 forretransmission purposes or will release the memory if an ACK has beenreceived.

Referring now to FIG. 5, the flow chart illustrates in more detail theprocess performed by the receiver. In step 500 the receiver receives acode word which is then stored in step 510. When code words have beenpreviously transmitted, the received and stored code word may becombined with a previously transmitted code word of the same data unit,in step 520. It is then decided in step 530 whether the PDU cansuccessfully be decoded. If so, a positive acknowledge message ACK issent back to the transmitter and all the stored code words of that PDUare released (step 540). Otherwise, a negative acknowledgement messageNAK is sent (step 550) to request a retransmission.

Depending on the bits that are retransmitted, three different types ofARQ can be distinguished:

Type I: The erroneous PDU's are discarded and a new copy of the PDU isretransmitted and decoded separately. There is no combining of earlierand later versions of that PDU.

Type II: The erroneous PDU that needs to be retransmitted is notdiscarded, but is combined with some incremental redundancy bitsprovided by the transmitter for subsequent decoding. Retransmitted PDU'ssometimes have higher coding rates and are combined at the receiver withthe stored values. Thus, only little redundancy is added in eachretransmission.

Type III: This ARQ type differs from Type-II ARQ only in that everyretransmitted PDU is now self decodable. This implies that the PDU isdecodable even without forming the combination with previous PDU's. Thisis useful if some PDU's are so heavily damaged that almost noinformation is reusable.

The schemes II and III are more intelligent and show some performancegain because they have the ability to adjust the coding rate to changingradio environments and to reuse redundancy of previously transmittedPDU's. Such Type II/III ARQ schemes are in the following referred to as“incremental redundancy”. The separate versions of the PDU's are encodeddifferently in the physical layer to increase the coding gain for thecombining process. These different portions of the overall code will becalled code blocks or code words.

As the ARQ schemes II and III put severe requirements on the memory sizeto store the soft decision values for subsequent combining, it has beenproposed to introduce a very fast feedback channel. The feedback channelis used for sending the ACK and NAK information from the receiver to thetransmitter. Usually, there is some round trip delay involved until anACK or a NAK can be sent because this information is gathered in statusreports. It has therefore been seen beneficial to send the feedback veryfast by the physical layer directly without an involvement of higherlayers such as Radio Link Control RLC. If a NAK has been received, thetransmitter can send the next code block with a minimum delay. Thus thenumber of code blocks that have to be stored are kept very small and theoverall delay is decreased.

Because of the limited spectrum resources, future mobile communicationsystems will be adaptive to the radio environment. The transmissionparameters such as modulation, data rate, spreading factor, and thenumber of spreading codes will be based on the current channelconditions.

However, in Frequency Division Duplex FDD systems the transmitterusually have only little knowledge of the channel conditions experiencedby the receiver. If there is some traffic from the receiver to thetransmitter on the reverse link, measurements on this traffic will notbe reliable since they are made on a different frequency. There are alsomeasurements that can be made in the transmitter. One of suchmeasurements e.g. for the node B is the transmitted code power whichcorresponds to the transmitted power to a certain User Equipment UE 120.Since the transmit power is controlled by UE power control commands, itfollows the channel conditions and tries to compensate for channelattenuation such as pathloss and fading. Nevertheless, such transmittermeasurements might not be meaningful under certain conditions.

Prior art adaptation techniques are often based on measurement reportsthat have to be sent from the receiver to the transmitter. RadioResource Control RRC can configure such kind of measurements that willthen be reported from the UE 120 to the Radio Network Controller RNC210. These measurement reports introduce additional signalling overheadthat has to be transmitted over the air. A continuous measurementreporting is therefore disadvantageous for adaptation purposes since itintroduces too much interference on the reverse link. On the other side,if reporting is not done continuously there will be a delay at thetransmitter so that adaptation cannot be performed accurately accordingto the present channel conditions.

Another prior art adaptation technique in simple ARQ systems is based onACK/NAK transmissions which are already available in the transmitter. Ifa high number of NAK messages are received, the transmitter can forinstance reduce the code rate. In systems using incremental redundancythis is however disadvantageous because hybrid ARQ Type II/IIIinherently involves a high number of retransmissions, i.e. a high numberof NAK messages.

While incremental redundancy can already be considered as an adaptivecoding scheme there is still a need for further adaptation. In thefollowing one example is given why further adaptation is still needed.

For mobile communication systems which do not use incrementalredundancy, the coding rate is usually around ½ and ⅓. Type II/IIIschemes use lower code rates for the first code block. Prior art systemsfor incremental redundancy are using a fixed coding rate for each codeblock. For instance, each code block could have a fixed code rate of 1.Assuming no acknowledgement, the overall coding rate (after subsequentcombining) will decrease with each retransmission to r=1, ½, ⅓, ¼ and soon.

Thus, compared to Type I ARQ; incremental redundancy schemes have moreretransmissions because the redundancy added per code block (with eachretransmission) is smaller in Type II/III schemes. For a good adaptationgranularity (coding rate to the channel condition) the code rate of asingle code block should be high. The number of retransmissions willincrease but the overall code rate will be nearer the optimum codingvalue at this particular moment.

There are several problems that come up with such design criteria:

First of all the number of retransmissions that have to be requestedover the feedback channel is large and leads to an increase of thesignalling overhead on the feedback channel. Further, the delay until awhole PDU is successfully decoded increases by the round trip delay RTDtime with each retransmission. Furthermore, the memory requirementsincrease with the number of retransmissions that are proportional to thetime needed for storing single code blocks. Moreover, if a high coderate is assumed for the first code block, e.g. r=1, there will be near100% retransmission in bad channel conditions because this data rate isdesigned for good channel conditions. On the other hand, if a low coderate is assumed for the first code block, e.g. r=½, there will be fewerretransmissions, but the gain will be relatively small compared tohybrid Type I ARQ.

Besides incremental redundancy, there are other known techniques ofadaptive coding. However, these prior art adaptive coding schemes do notconsider the behaviour of the ARQ Type II/III scheme where the code canbe split into multiple code blocks. Since there will be a gain from thetime diversity of the multiple code blocks the required coding rate forhybrid ARQ Type I scheme will be different from hybrid ARQ Type II/IIIschemes.

SUMMARY OF THE INVENTION

In order to overcome the above discussed drawbacks of the prior artsystems, it is the object of the invention to provide a hybrid ARQtechniques allowing for additional measurements in the transmitterwithout the need of signalling measurement reports on the reverse link,thereby reducing the overhead.

This object is solved by the invention as claimed in the independentclaims.

The invention is advantageous in that it provides an adaptive codingscheme with incremental redundancy, making use of information that isalready available at the transmitter. Thus, the invention provides atransmitter measurement overcoming the numerous drawbacks of the priorart schemes.

Preferred embodiments are defined in the dependent claims.

As mentioned above incremental redundancy schemes inherently receiveACK/NAK information on the feedback channel. As such, it is not veryexpressive since a high number of NAK messages have to be received for agood adaptation. In the preferred embodiments, the invention provides atechnique to derive the overall code rate that was needed for theacknowledgement of previous PDUs. This information can be derived bysumming up the number of retransmissions with the corresponding coderate of the corresponding PDUs. This allows for instance to determine anestimate of the required FEC parameter without the need for anyadditional measurement reports from the receiver to the transmitter.

Besides the new transmitter measurement, the present invention makes, inone embodiment, use of this measurement for adaptation. Depending on thechannel conditions, the number of retransmissions needed for decodingthe PDU's will vary. For incremental redundancy the performance gaincompared to ARQ Type I is only large if there is a certain portion ofNAK messages at the first transmission (e.g. larger than 50%). For goodchannel conditions a code rate near 1 (i.e. no redundancy) might beappropriate while in very bad situations (e.g. fading, shadowing) evencode rates of ⅓ could allow only some packets to be decoded correctly.

The invention further provides an adaptive coding for the code rates ofthe separate code blocks. Particularly the code rate of the first codeblock can be tailored to deliver a certain target rate ofretransmissions under several channel conditions.

The invention will now be described in more detail with reference to theaccompanying drawings in which:

FIG. 1 illustrates the UMTS architecture;

FIG. 2 illustrates the UTRAN architecture;

FIG. 3 is a block diagram illustrating a transmitter capable of FECencoding the data to be transmitted;

FIG. 4 is a block diagram illustrating a transmitter adapted to theFEC/ARQ scheme;

FIG. 5 is a flowchart illustrating the operation of a receiver workingaccording to the ARQ technique;

FIG. 6 is a block diagram illustrating a transmitter according to afirst preferred embodiment of the present invention;

FIG. 7 is a flow chart illustrating the process of operating thetransmitter according to FIG. 6;

FIG. 8 is a block diagram illustrating a transmitter according to asecond preferred embodiment of the present invention; and

FIG. 9 is a flow chart illustrating the process of operating thetransmitter according to FIG. 8.

DETAILED DESCRIPTION OF EMBODIMENTS

The invention will now be described in more detail with reference to theaccompanying drawings.

The code rate r is the ratio of the number of information bits k to thenumber of transmitted coded bits n. Different code rates and code wordscan easily be generated using rate compatible punctured codes such asRCPT and RCPC codes. The code words are punctured from a common mothercode. The mother code should have a low coding rate to allow severalhigh code rate code words to be generated from it. Either the PDU or themother code needs to be stored in the transmitter for the caseretransmissions are requested and new code words are needed to be sent.While it will be understood by those of ordinary skill in the art thatthe invention is not limited to the specific embodiments describedhereafter, it will now be assumed that the whole mother code (i.e. allcode words) is stored in the transmitter for retransmissions. Thisreduces the complexity for encoding but needs more memory in thetransmitter.

Referring now to FIG. 6 which illustrates a first preferred embodimentof a transmitter according to the present invention, the arrangementdiffers from that of FIG. 4 in that there is provided a NAK counter 600and a measurement unit 610. The operation of the transmitter will now beexplained in more detail with reference to FIG. 7.

After encoding the PDUs in steps 700 by the FEC encoder 310, the codewords are buffered in the code word buffer 320 (step 710). Then, thefirst code word is transmitted in step 720. If the receiver is not ableto decode the data, it will send a NAK message to request thetransmitter to send the next new code to add more redundancy to thereceived data. In this case, the transmitter will receive in step 730the NAK message and will select the next code word from code word buffer320 in step 740. This next code word is then queued for transmission.After reception, the receiver will combine the new code word with thealready received code word so that the overall code rate in the receiveris r_(tot)=k/(n_(cw) ₁ +n_(cw) ₂ ). Thus, redundancy is added and it ismore likely that the packet can be decoded.

If the transmitter receives in step 730 an ACK message, all the codewords of the respective PDU can be cleared in step 760.

As shown in the flow chart, the NAK counter 600 is incremented in step750 whenever a NAK message is received. Thus, the NAK counter 600 willcount all NAK messages of a PDU until the PDU has been acknowledged.With the knowledge of the code rates of each code word the numberN_(NAK) of retransmission per PDU can be used in the measurement unit610 to calculate the overall PDU code rate, the average number ofretransmissions per PDU or the average number of retransmissions percode word. If the code rates of the code words are fixed the measurementunit 610 will preferably contain a memory for storing the code rates ofeach code word. As will be described in more detail below, themeasurement can be averaged over a number of PDUs or over a certaintime. For this purpose, the measurement 610 is preferably provided witha filter function. Averaging is preferably applied depending on theround trip delay until retransmissions can be sent and depending on howfast channel conditions change. In the filter function also a weightingcould be applied in unit 610 to give the recent measurement more weightcompared to previous measurements.

In the preferred embodiment, the measurement unit 610 reports themeasurement value to higher layer units or other entities. Preferably,the measurement value is reported to the base station (Node B) 220 or tothe Radio Network Controller RNC 210. Reporting can be done on demand,periodically or based on certain thresholds.

Examples of measurement values which can be calculated in step 770 bythe transmitter based on the number of retransmissions without anyadditional measurements, will now be described in more detail. It willbe appreciated that due to the inherent behaviour of hybrid ARQ TypeII/III it is possible to derive the overall coding rate that was neededfor successfully decoding the PDU's. Any adaptation to the radioenvironment can be based on such measurement values.

A first example of calculating a measurement value is to calculate theoverall coding rate. For this purpose the transmitter counts the numberN_(NAK) of retransmissions per PDU and the coding rates r_(cw) _(i) ofeach code block if this cannot be derived explicitly. The overall PDUcoding rate r_(tot) can now be calculated according to the followingequation$r_{tot} = {\frac{k}{n_{{cw}_{1}} + n_{{cw}_{2}} + \ldots + n_{{cw}_{N_{NAK}}}} = {\frac{k}{\sum\limits_{i = 1}^{N_{NAK}}\; n_{{cw}_{i}}} = \frac{1}{\sum\limits_{i = 1}^{N_{NAK}}\frac{1}{r_{{cw}_{i}}}}}}$where n_(cw) _(i) is the number of coded bits of code word i. Dependingon the accuracy and the time variance of the mobile channel, the coderate can be averaged$\overset{\_}{r_{tot}} = {\frac{1}{N_{PDU}} \cdot {\sum\limits_{i = 1}^{N_{PDU}}\; r_{{tot}_{i}}}}$where N_(PDU) is the number of packets used for averaging. In apreferred embodiment, the code rate of the first code block is adaptedaccording to the average coding rate that has been used for previouslytransmitted PDU's.

Another example of calculating a measurement value is to calculate theaverage number of retransmissions per PDU. For each PDU, the number ofretransmissions needed for successfully decoding is N_(NAK) _(i) . Theaverage can then be calculated according to$\overset{\_}{N_{NAK}} = {\frac{1}{N_{PDU}} \cdot {\sum\limits_{i = 1}^{N_{PDU}}N_{{NAK}_{i}}}}$

This value is zero when there was no retransmission. In a preferredembodiment, the code rate of the first code block is adapted accordingto the number of retransmissions needed per PDU. Compared to ARQ Type Iadaptive coding schemes the coding will not be adapted to have noretransmissions (i.e. N_(NAK)=0) but will ensure to receive a specifiedaverage number of retransmissions. Preferably, the average number isabout 1 or even larger.

Yet another example of calculating a measurement value is to calculatethe average number of transmissions per code word. The average number ofretransmissions (equal to all NAK messages) per code word×averaged overN_(PDU) can be calculated according to$\overset{\_}{N_{NAKcwx}} = {\frac{1}{N_{PDU}} \cdot {\sum\limits_{i = 1}^{N_{PDU}}N_{{NAKcwx}_{i}}}}$

While in the above discussion the transmitter measurement of the presentinvention has been described in more detail, the following discussionwill focus on using this measurement for adaptation purposes. Referenceis made in this respect to FIG. 8 which differs from the arrangement ofFIG. 6 by showing the FEC control unit 800. The transmitter of FIG. 8performs adaptation of the FEC parameter used for encoding the data inthe FEC encoder 310.

In FIG. 8, the adaptation is carried out by the FEC control unit 800.Based on the measurement provided by the measurement unit 610, the FECcontrol unit 800 adapts the FEC parameter. Once adaptation is carriedout, the new code rates for the code words are reported to themeasurement unit 610. The process of performing the adaptation isillustrated in FIG. 9.

The initial FEC values which are determined in step 900, are preferablybased on measurements that have been made before transmission. Inanother preferred embodiment, the initial parameters are set to highcode rates. During transmission the FEC parameters will be adapted tothe environment. For this purpose, certain parameters used as adaptationcriteria have to be monitored. This can for instance be the number ofoverall retransmissions per PDU, the number of retransmissions per codeword, or the overall code rate (see above). It will however beappreciated by those of ordinary skill in the art that the adaptationcan also be based on any other parameter measured in the transmitter orreceived from the receiver.

In the adaptation process depicted in FIG. 9, thresholds are determinedin step 920. Some variance can be included to avoid too frequentswitching between different FEC parameter sets. The parameter monitoredin step 910 is then compared to the threshold in steps 930 and 950. Ifthe monitored parameter such as the average number of retransmission percode word is larger than the threshold, the code rate will be decreased.If on the other hand the monitored parameter is less than the threshold,the code rate will be increased.

In a preferred embodiment, the adaptation process is limited to some oronly one of the code words. If only the first code word is adapted thecode rate (or FEC parameter) of the following code words can be fixed toa higher coding rate. This ensures a small granularity of the overallcode rate and the overall coding rate near the optimum. Alternatively,the code rate of the following code words can be explicitly derived fromthe coding rate of the first code block. This reduces the signallingoverhead because the receiver can derive the coding rates of subsequentcode blocks from the coding rates of the first code block.

In another preferred embodiment, the code rates of the code blocks areadapted to allow for decoding only a certain percentage of the PDU's perretransmission correctly. This percentage is preferably 25%.

Referring back to FIG. 8, instead of or in addition to the adaption ofencoding parameters, in further preferred embodiments of the inventionthere will be a transmission parameter adapted. In one preferredembodiment, the modulation form is adapted. For this purpose, thecontrol unit 800 has additional access (not shown in FIG. 8) tomodulator 330. In another preferred embodiment, the adapted transmissionparameter is the spreading factor or the number of spreading codes.Then, the control unit 800 has additional access (not shown in FIG. 8)to spreader 340.

1. A hybrid ARQ method for transmitting a data unit on a radio channel,the method comprising: encoding the data unit into at least a first codeword and a second code word using at least one encoding parameter;transmitting said first code word using a transmitting parameter;receiving a positive (ACK) or negative (NAK) acknowledgement messagefrom the receiver indicating that the data unit was transmittedsuccessfully or not; and if a negative acknowledgement message wasreceived, transmitting said second code word, wherein at least one ofsaid encoding parameter and said transmitting parameter is based on ameasurement value indicating the quality for the first code word.
 2. Themethod of claim 1, wherein said measurement value is the number(N_(NAKi)) of negative acknowledgment messages received until a positiveacknowledgment message is received.
 3. The method of claim 1, whereinsaid measurement value is the overall coding rate (r_(tot)) determinedfrom the coding rates (r_(CWi)) of each code word which has beentransmitted until the positive acknowledgment message is received. 4.The method of claim 1 wherein said measurement value is the number(N_(NAKCWXi)) of negative acknowledgment messages per code word (x). 5.The method of claim 1 further comprising the step of averaging themeasurement value over a number (N_(PDU)) of data units or applying aweighted 10 filter function giving less weight to previous data units.6. The method of claim 1 wherein said at least one encoding parameter isa FEC code rate.
 7. The method of claim 1 further comprising, adaptingat least one encoding parameter to the measurement value.
 8. The methodof claims 1 further comprising, adapting a transmission parameter to themeasurement value.
 9. The method of claim 8 wherein said transmissionparameter is the modulation form.
 10. The method of claim 8 wherein saidtransmission parameter is the spreading factor.
 11. The method of claim8 wherein said transmission parameter is the number of spreading codes.12. The method of claim 1 wherein said at least one encoding parameteris the code rate of the code words, and the method further comprisesinitializing the code rates to high values.
 13. The method of claim 12wherein said code rates of the code words following the first code wordare derived from the code rate of the first code word.
 14. The method ofclaim 12 further comprising signaling only the code rate of the firstcode word to the receiver.
 15. A hybrid ARQ transmitter for transmittinga data unit on a radio channel, comprising: an encoder for encoding thedata unit into at least a first code word and a second code word usingat least one encoding parameter; a transmission unit for transmittingsaid first code word using a transmitting parameter; a receiving unitfor receiving a positive (ACK) or negative (NAK) acknowledgement messagefrom the receiver indicating that the data unit is transmittedsuccessfully or not; and an operating unit that operates thetransmission unit for transmitting said second code word if a negativeacknowledgement message was received, wherein at least one of saidencoding parameter and said transmitting parameter is based on ameasurement value indicating the quality for the first code word. 16.The transmitter of claim 15 arranged for performing the method ofclaim
 1. 17. The transmitter of claim 15, being a base station.
 18. Thetransmitter of claim 17 further comprising means for sending themeasurement value to a radio network controller on demand.
 19. Thetransmitter of claim 17 further comprising means for sending themeasurement value to a radio network controller periodically.
 20. Thetransmitter of claim 17 further comprising means for sending themeasurement value to a radio network controller event triggered.
 21. Asystem comprising a transmitter according to claim 15 and a receiver.22. The transmitter of claim 16, being a base station.
 23. Thetransmitter of claim 18 further comprising means for sending themeasurement value to a radio network controller periodically.
 24. Thetransmitter of claim 22 further comprising means for sending themeasurement value to a radio network controller on demand.
 25. Thetransmitter of claim 22 further comprising means for sending themeasurement value to a radio network controller event triggered.
 26. Themethod of claim 1, wherein said measurement value is used to reach atarget error rate.
 27. The method of claim 1, wherein the first codeword is different from the second code word with respect to theredundancy.